1. Field of the Invention
The present invention relates to a thin film field effect transistor and a display using the same. Particularly, it relates to a thin film field effect transistor in which an amorphous oxide semiconductor is used for an active layer, and a display using the same.
2. Description of the Related Art
In recent years, flat panel displays (FPDs) have been put to practical use, due to the progress made in liquid crystal and electroluminescence (EL) technologies, etc. Especially, an organic electroluminescence element (hereinafter referred to as an “organic EL element” in some cases) formed using a thin film material which emits light by excitation due to application of electric current can provide light emission of high brightness at low voltage, and thus is expected to obtain effects such as reduction in device thickness, weight, and size, and power saving in wide ranging applications including mobile phone displays, personal digital assistants (PDA), computer displays, car information displays, TV monitors, and general illumination.
These FPDs are driven by an active matrix circuit including thin film field effect transistors each using, as an active layer, an amorphous silicon thin film or a polycrystalline silicon thin film provided on a glass substrate. (In the description below, the thin film field effect transistor is sometimes referred to as a “thin film transistor” or “TFT”.)
On the other hand, to make the FPD thinner, lighter, and more resistant to breakage, attempts are being made to use a resin substrate which is light in weight and flexible, instead of the glass substrate.
However, fabrication of the transistors using the silicon thin films described above requires a thermal treatment process at a relatively high temperature, and it is difficult to form the transistors directly on resin substrates, which are generally low in heat resistance.
For example, a MOSFET (metal-oxide semiconductor field effect transistor), which reduces a driving voltage of a transistor that uses a silicon thin film, is disclosed, and a configuration in which indium tin oxide (ITO), tin oxide or zinc oxide is used as a semiconductor material of an active layer and a dielectric material having a large dielectric constant is used in a gate insulating layer is disclosed. However, ITO, tin oxide or zinc oxide is a crystalline metal oxide. In order to obtain desired semiconductor characteristics using the crystalline metal oxide in an active layer, it is necessary to conduct a high temperature heat treatment step for regulation of the crystallization, such as post annealing at 300° C. for 15 minutes, after film formation by sputtering. Accordingly, such an active layer is difficult to form directly on a resin substrate that is poor in heat resistance.
Amorphous oxides, such as In—Ga—Zn—O-based amorphous oxides, can form a film at low temperatures, and have been attracting attention as a material that can form a film on a plastic film at room temperature.
However, a TFT using an amorphous oxide semiconductor has problems of high OFF current and low ON/OFF ratio.
Further, it is reported that when a TFT in which an amorphous oxide semiconductor is used as an active layer contacts with epoxy resin or acrylic resin, which is a common protective layer material, and repeatedly driven, the TFT exhibits unstable behavior in that the threshold voltage for allowing a current to flow varies each time. In order to prevent this, for example, Japanese Patent Application Laid-Open (JP-A) No. 2007-299913 discloses a method of covering the active layer with an inorganic insulating layer or a fully fluorinated resin to avoid direct contact of the active layer with a protective layer.
Further, JP-A No. 2007-73705 mentions a problem in that a TFT having an active layer formed of an amorphous oxide semiconductor exhibits changes in characteristics due to the influence of gas components (oxygen gas and nitrogen gas) and moisture in an atmosphere during storage, and discloses covering the channel portion with a protective layer constituted of a silicon resin or the like.